The prior art considered in conjunction with the preparation of this patent application is U.S. Pat. No. 3,472,829; 3,732,335 and 3,776,979. These patents and any other publications disclosed herein are to be considered as incorporated, in toto, herein by reference for all intents and purposes.
The present invention relates to a process for the preparation of polypropylene modified by a copolymer of propylene and another alpha-olefin.
Isotactic polypropylene is known to have excellent mechanical characteristics at room temperature and above; however, at temperatures below 5.degree. C., its impact resistance is rather poor. A block or heterblock copolymer constituted by segments of a random ethylene/propylene copolymer and by segments of isotactic polypropylene which contains relatively little ethylene has much better mechanical properties at low temperature than those of isotactic polypropylene. On the other hand, at room temperature or above, the physical and mechanical characteristics of such a copolymer are modified only slightly with respect to those of isotactic polypropylene.
The preparation of block or heterblock copolymers or at least one alpha-olefin by alternating polymerization is well known. This process can be carried out by modifying the composition and nature of the monomer(s) contacted with the polymerization catalyst as has been disclosed by British Pat. No. 838,996 of Dec. 27, 1955, issued to Goodrich Gulf Chemicals, French Pat. No. 1,018,971 of May 5, 1950, and Belgian Pat. No. 560,366 of Aug. 28, 1957 issued to Bataafsche Petroleum.
The known technique for preparing modified polypropylene generally involves preparing a product which is comprised mainly of isotactic polypropylene and segments of an ethylene/propylene copolymer having random distribution, as disclosed in French Pat. No. 1,220,947 of Jan. 8, 1959, issued to Phillips Petroleum Company and French Pat. No. 1,220,573 of Apr. 16, 1959, issued to Montecatini. Similar products may also be obtained from butene-1 and another higher alpha-olefin. "Ziegler" type catalysts are generally used for these processes.
It is know that propylene polymers which are modified by randon propylene/second alpha-olefin copolymers may be prepared in two different ways, depending on the order chosen for the introduction of the monomers: homopolymer, then the random propylene/second alpha-olefin copolymer; or random propylene/second alpha-olefin copolymer, then homo-polymer.
In French Pat. No. 1,294,608 of July 10, 1961, issued to Hoechst (this patent and each of the foregoing mentioned patents are incorporated by reference), the preparation of a polyolefin with an improved impact resistance is described in which:
1. A copolymer is prepared by polymerizing a mixture of 75 to 30% by volume of ethylene and 25 to 70% by volume of propylene or butene-1 in the presence of 0.5 to 5 millimoles of TiCl.sub.3 per liter of dispersant and 0.5 to 10 millimoles of triethylaluminum per liter of dispersant to obtain up to 20% by weight of the total misture of polymers, and
2. After addition of 5 to 10 supplemental millimoles of TiCl.sub.3 per liter of dispersant and 10 to 30 millimoles of diethylaluminum monochloride per liter of dispersant, a homopolymer is prepared by polymerizing olefins having from 2 to 10 carbon atoms, this homopolymer representing 98 to 80% by weight of the total mixture of polymers.
The triethylaminum activator promotes the formation of an amorphous copolymer, whereas diethylaluminum monochloride promotes the formation of a highly crystalline compound.
The foregoing process (French Pat. No. 1,294,608) wherein a random copolymer is produced first and then an olefin is homopolymerized to produce a homopolymer chain has numerous disadvantages.
When a random propylene/second alpha-olefin copolymer is prepared first, the second alpha-olefin must be eliminated before effecting the homopolymerization. In order to eliminate the alpha-olefin involved in the copolymerization, either the second alpha-olefin must be expanded or the reaction must be extended until the entire quantity of the second alpha-olefin is consumed. Moreover, the presence of residual quantites of triethylaluminum in the second step of such a process influences homopolymerization, favoring the formation of a poorly crystalline homopolymer.
The formation of an amorphous copolymer in an inert reaction medium, according to the process of the above-mentioned French Pat. No. 1,294,608, also involves numerous technological difficulties due to the dissolution of the resultant copolymer in the reaction medium.